Semi-insulating silicon carbide without vanadium domination

ABSTRACT

A semi-insulating bulk single crystal of silicon carbide is disclosed that has a resistivity of at least 5000 Ω-cm at room temperature and a concentration of trapping elements that create states at least 700 meV from the valence or conduction band that is below the amounts that will affect the resistivity of the crystal, preferably below detectable levels. A method of forming the crystal is also disclosed, along with some resulting devices that take advantage of the microwave frequency capabilities of devices formed using substrates according to the invention.

FIELD OF THE INVENTION

This is a continuation-in-part of Ser. No. 09/757,950 filed Jan. 10,2001, which is a continuation of Ser. No. 09/313,802 filed May 18, 1999,now U.S. Pat. No. 6,218,680. The present invention relates to the growthof high quality silicon carbide crystals for specific purposes, and inparticular relates to the production of high quality semi-insulatingsilicon carbide substrates that are useful in microwave devices.

This invention was made under Department of the Air Force Contract No.F33615-95-C-5426. The government may have certain rights in thisinvention.

BACKGROUND OF THE INVENTION

The term “microwaves” refers to electromagnetic energy in frequenciescovering the range of about 0.1 gigahertz (GHz) to 1,000 GHz withcorresponding wavelengths from about 300 centimeters to about 0.3millimeters. Although “microwaves” are perhaps most widely associated bythe layperson with cooking devices, those persons familiar withelectronic devices recognize that the microwave frequencies are used fora large variety of electronic purposes and in corresponding electronicdevices, including various communication devices, and the associatedcircuit elements and circuits that operate them. As is the case withmany other semiconductor electronic devices and resulting circuits, theability of a device (or circuit) to exhibit certain desired or necessaryperformance characteristics depends to a large extent, and oftenentirely, upon the material from which it is made. One appropriatecandidate material for microwave devices is silicon carbide, whichoffers a primary advantage for microwave applications of a very highelectric breakdown field. This characteristic of silicon carbide enablesdevices such as metal semiconductor field effect transistors (MESFETs)to operate at drain voltages ten times higher than field effecttransistors formed in gallium arsenide (GaAs).

Additionally, silicon carbide has the significant advantage of a thermalconductivity of 4.9 watts per degree Kelvin per centimeter (W/K-cm)which is 3.3 times higher than silicon and ten times higher than eithergallium arsenide or sapphire. These properties give silicon carbide ahigh power density in terms of gate periphery measured in terms of wattsper millimeter (W/mm) and also an extremely high power handlingcapability in terms of die area (W/mm). This is particularlyadvantageous for high power, high frequency applications because diesize becomes limited by wavelength. Accordingly, because of theexcellent thermal and electronic properties of silicon carbide, at anygiven frequency, silicon carbide MESFETs should be capable of at leastfive times the power of devices made from gallium arsenide.

As recognized by those familiar with microwave devices, they oftenrequire high resistivity (“semi-insulating”) substrates for couplingpurposes because conductive substrates tend to cause significantproblems at microwave frequencies. As used herein, the terms “highresistivity” and “semi-insulating” can be considered synonymous for mostpurposes. In general, both terms describe a semiconductor materialhaving a resistivity greater than about 1500 ohm-centimeters (Ωcm).

Such microwave devices are particularly important for monolithicmicrowave integrated circuits (MMICs) which are widely used incommunications devices such as pagers and cellular phones, and whichgenerally require a high resistivity substrate. Accordingly, thefollowing characteristics are desirable for microwave device substrates:A high crystalline quality suitable for highly complex, high performancecircuit elements, good thermal conductivity, good electrical isolationbetween devices and to the substrate, low resistive losscharacteristics, low cross-talk characteristics, and large waferdiameter.

Given silicon carbide's wide bandgap (3.2 eV in 4H silicon carbide at300K), such semi-insulating characteristics should be theoreticallypossible. As one result, an appropriate high resistivity silicon carbidesubstrate would permit both power and passive devices to be placed onthe same integrated circuit (“chip”) thus decreasing the size of thedevice while increasing its efficiency and performance. Silicon carbidealso provides other favorable qualities, including the capacity tooperate at high temperatures without physical, chemical, or electricalbreakdown.

As those familiar with silicon carbide are aware, however, siliconcarbide grown by most techniques is generally too conductive for thesepurposes. In particular, the nominal or unintentional nitrogenconcentration in silicon carbide tends to be high enough in sublimationgrown crystals (1-2×10¹⁷ cm⁻³) to provide sufficient conductivity toprevent such silicon carbide from being used in microwave devices.

In order to be particularly useful, silicon carbide devices should havea substrate resistivity of at least 1500 ohm-centimeters (Ω-cm) in orderto achieve RF passive behavior. Furthermore, resistivities of 5000 Ω-cmor better are needed to minimize device transmission line losses to anacceptable level of 0.1 dB/cm or less. For device isolation and tominimize backgating effects, the resistivity of semi-insulating siliconcarbide should approach a range of 50,000 Ω-cm or higher. Present worktends to assert that the semi-insulating behavior of a silicon carbidesubstrate is the result of energy levels deep within the band gap of thesilicon carbide; i.e., farther from both the valence band and theconduction band than the energy levels created by p-type and n-typedopants; e.g., U.S. Pat. No. 5,611,955. According to the '955 patent,the deep levels in the silicon carbide between the valence andconduction bands can be produced by the controlled introduction ofselected elements such as transition metals or passivating elements suchas hydrogen, chlorine or fluorine, or combinations of these elementsinto the silicon carbide to form the deep level centers in the siliconcarbide; e.g., column 3, lines 37-53. See also, Mitchel, The 1.1 eV DeepLevel in 4H-SiC. SIMC-X, Berkley Calif., Jun. 1998; Hobgood,Semi-Insulating GH-SiC Grown by Physical Vapor Transport, Appl. Phys.Lett. Vol. 66, No. 11 (1995); WO 95/04171; Sriram, RF Performance of SiCMESFETs on High Resistivity Substrates, IEEE Electron Device Letters,Vol. 15, No. 11 (1994); Evwaraye, Examination of Electrical and OpticalProperties of Vanadium in Bulk n-type Silicon Carbide, J. Appl. Phys. 76(10) (1994); Schneider, Infrared Spectra and Electron Spin Resonance ofVanadium Deep Level Impurities in Silicon Carbide, Appl. Phys. Lett.56(12) (1990); and Allen, Frequency and Power Performance of MicrowaveSiC FET's, Proceedings of International Conference on Silicon Carbideand Related Materials 1995, Institute of Physics.

Further to the conventional thinking, these deep level elementalimpurities (also known as deep level trapping elements) can beincorporated by introducing them during high temperature sublimation orchemical vapor deposition (CVD) growth of high purity silicon carbide.In particular, vanadium is considered a desirable transition metal forthis purpose.

According to the '955 patent and similar art, the vanadium compensatesthe silicon carbide material and produces the high resistivity (i.e.,semi-insulating) characteristics of silicon carbide.

The introduction of vanadium as a compensating element to producesemi-insulating silicon carbide, however, also introduces certaindisadvantages. First, the presence of electronically significant amountsof any dopant, including vanadium, can negatively affect the crystallinequality of the resulting material. Accordingly, to the extent thatvanadium or other elements can be significantly reduced or eliminated,the crystal quality of the resulting material, and its correspondingelectronic quality, can be increased. In particular, the presentunderstanding is that compensating amounts of vanadium can cause growthdefects such as inclusions and micropipes in silicon carbide.

As a second disadvantage, the introduction of compensating amounts ofvanadium can reduce the yield and add expense to the production ofsemi-insulating silicon carbide substrates. Third, the proactivecompensation of silicon carbide, or any other semiconductor element, canbe somewhat complex and unpredictable and thus introduces manufacturingcomplexity that can be desirably avoided if the compensation can beavoided.

In parent application Ser. No. 09/313,802 filed May 18, 1999 and itscontinuation Ser. No. 09/757,950 filed Jan. 10, 2001, an improvedsemi-insulating silicon carbide is disclosed in which the concentrationof vanadium is maintained below detectable (e.g., SIMS-detectable)levels in compensated silicon carbide single crystals. In describing therelevant doping, the '802 application, along with much of the prior art,occasionally refers to particular dopants as being “deep,” or “shallow.”Although the terms “deep” and “shallow” can have illustrative value indescribing the state and energy levels associated with certain dopants,they are best understood in a relative rather than limiting sense.

For example, in some circumstances, a level 300 meV or more from theband edge is referred to as “deep.” Some elements, however, that producelevels in that range (e.g., boron) can also act in “shallow” fashion;i.e. they can produce a conductive level rather than a level that raisesthe resistivity. Furthermore, and as the case with boron (B), individualelements can produce more than one level within the bandgap.

OBJECT AND SUMMARY OF THE INVENTION

Therefore, it is an object of the present invention to provide asemi-insulating silicon carbide substrate without characterizingparticular dopants as being universally “deep” or “shallow,” and toprovide a semi-insulating silicon carbide substrate that offers thecapabilities that are required and advantageous for high frequencyoperation, but while avoiding the disadvantages of prior materials andtechniques.

The invention meets this object with a semi-insulating bulk singlecrystal of silicon carbide having a resistivity of at least 5000 Ω-cm atroom temperature and a concentration of transition elements that isbelow 1E16.

In another aspect, the invention is a semi-insulating bulk singlecrystal of silicon carbide having a resistivity of at least 5000ohm-centimeters at room temperature and a concentration of trappingelements that create states at least 700 meV from the valence orconduction band that is below the amount that affects the electricalcharacteristics of the crystal.

In yet another aspect, the invention comprises devices that incorporatethe semi-insulating silicon carbide according to the claimed invention,including MESFETs, certain MOSFETs, and HEMTs (High Electron MobilityTransistors).

The foregoing and other objects and advantages of the invention and themanner in which the same are accomplished will become clearer based onthe following detailed description taken in conjunction with theaccompanying drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 3 are the plots of the Hall effect measurements carriedout on wafers made according to the present invention.

FIG. 4 is a plot of the natural log of the carrier concentration againstthe reciprocal temperature (degrees Kelvin) for semi-insulating siliconcarbide according to the present invention.

FIG. 5 is a plot of the natural log of the resistivity as againstreciprocal temperature for semi-insulating silicon carbide according tothe present invention;

FIGS. 6 through 8 are the same measurements as represented in FIGS. 1through 3, but taken from a different portion of the substrate wafer;

FIG. 9 is another plot of the natural log that carrier concentrationversus reciprocal temperature for the samples illustrated in FIGS. 6through 8;

FIG. 10 is another plot of the natural log of resistivity versusreciprocal temperature and again corresponding to the samplemeasurements of FIGS. 6 through 8;

FIGS. 11 through 13 are yet another set of plots identical to FIGS. 1through 3 and 6 through 8 for yet another measurement on a differentportion of the semi-conducting silicon carbide material;

FIG. 14 is another plot of the natural log of resistivity as againstreciprocal temperature for the samples illustrated in FIGS. 11 through13; and

FIGS. 15, 16, and 17 are plots of Secondary Ion Mass Spectroscopy (SIMS)for various samples of materials according to the present invention andprior art material.

DETAILED DESCRIPTION

In a first embodiment, the invention is a semi-insulating bulk singlecrystal of silicon carbide having concentration of transition elementsthat is below a level at which such elements dominate the resistivity ofthe crystal and preferably at a concentration that is below 10¹⁶ percubic centimeter (cm⁻³); i.e., 1E16.

In another embodiment, the invention is a semi-insulating bulk singlecrystal of silicon carbide having a resistivity of at least 5000ohm-centimeters at room temperature and a concentration of trappingelements that create states at least 700 meV from the valence orconduction band that is below the amount that affects the electricalcharacteristics of the crystal.

As used herein the term “transition element” refers to those elementsfrom the periodic table which, when incorporated as dopants in siliconcarbide form states at levels between the valence and conduction bandsof silicon carbide that are much farther removed from both theconduction and valence bands than are more conventional p-type or n-typedopants. As set forth in the Field and Background, vanadium is a commontransition element with such characteristics.

As further used herein, the concentration that is defined as “belowdetectable levels,” refers to elements that are present in amounts thatcannot be detected by modern sophisticated analytical techniques. Inparticular, because one of the more common techniques for detectingelements in small amounts is secondary ion mass spectroscopy (“SIMS”),the detectable limits referred to herein are those amounts of elementssuch as vanadium and other transition metals that are present in amountsof less than 1×10¹⁶ cm⁻³ (1E16), or in other cases, less than about1E14. These two amounts represent typical detection limits for mosttrace elements (particularly vanadium) using SIMS techniques; e.g., SIMSTheory—Sensitivity and Detection Limits, Charles Evans & Associates(1995), www.cea.com.

As noted above, vanadium (V) is one of the more common elements forproducing semi-insulating silicon carbide. Accordingly, the invention ischaracterized in that vanadium is either absent, or if present, ispresent in amounts below those which will substantially affect theresistivity of the crystal, and preferably below 1E16.

Although other polytypes (i.e., crystal structures) are possible, thesilicon carbide single crystal according to this embodiment of theinvention preferably has a polytype selected from the group consistingof the 3C, 4H, 6H and 15R polytypes.

Furthermore, in order to avoid the problems associated with the presenceof nitrogen, and the resulting necessity to attempt to compensate forthe nitrogen, silicon carbide single crystals according to thisembodiment of the invention preferably have a concentration of nitrogenatoms below about 1×10¹⁷ cm⁻³ (1E17). More preferably, the siliconcarbide semi-insulating single crystal according to the presentinvention will have a concentration of nitrogen of 5E16 or less. Theconcentration of vanadium is less than 1E16 atoms per cubic centimeter,and most preferably less than 1E14 atoms per cubic centimeter.Additionally, the resulting bulk silicon carbide single crystal willpreferably have a resistivity of at least 10,000 ω-cm at roomtemperature, and most preferably a resistivity of at least 50,000 ω-cmat room temperature.

For purposes of providing semi-insulating silicon carbide substrates forhigh frequency MESFETs, the 4H polytype is preferred for, its higherbulk electron mobility. For other devices, the other polytypes may bepreferred. Accordingly, one of the more preferred embodiments of theinvention is a semi-insulating bulk single crystal of 4H silicon carbidethat has a resistivity of at least 10,000 ω-cm at room temperature andconcentration of vanadium atoms of less than 1E14.

Preferably, the method of producing a semi-insulating bulk singlecrystal of silicon carbide comprises heating a silicon carbide sourcepowder to sublimation while, heating and then maintaining a siliconcarbide seed crystal to a temperature below the temperature of thesource powder, and at which temperature sublimed species from the sourcepowder will condense upon the seed crystal. Thereafter, the methodincludes continuing to heat the silicon carbide source powder until adesired amount of single crystal bulk growth has occurred upon the seedcrystal. The method is characterized in that (1) the amounts oftransition elements in the source powder (as described above) are belowthe relevant amounts, (2) the source powder contains 5E16 or lessnitrogen, and (3) during sublimation growth, the source powder and theseed crystal are maintained at respective temperatures high enough tosignificantly reduce the amount of nitrogen that would otherwise beincorporated into the bulk growth on the seed crystal and to increasethe number of point defects (sometimes referred to as intrinsic pointdefects) in the bulk growth on the seed crystal to an amount thatrenders the resulting silicon carbide bulk single crystalsemi-insulating. Preferably and conceptually, by keeping the amounts ofnitrogen or other dopants as low as possible, the number of pointdefects required to make the crystal semi-insulating can also beminimized. Presently, the preferred number of point defects appears tobe in the range of 1E15-5E17.

In order to produce the semi-insulating silicon carbide according to theinvention, the source powder that is used must be free of vanadium, orif vanadium is present, it must be below detectable levels. As notedabove, the detectable levels are typically characterized as those thatcan be measured using SIMS. Stated differently, the amount of vanadiumin the source powder is preferably less than 1E16 atoms per cubiccentimeter, and most preferably less than 1E14 atoms per cubiccentimeter.

In preferred embodiments, nitrogen is minimized by using high prioritygraphite as one of the starting materials for SiC, as well as by usingpurified graphite parts in the reactor itself. In general, graphite (forsource powder or reactor parts) can be purified of potential dopingelements such as boron or aluminum by heating in the presence of halogengases (e.g. Cl₂), and if necessary, by further heating (a “bake out”) inan inert atmosphere (e.g. Ar) at about 2500° C. Appropriate purificationtechniques are also known in the art (e.g. U.S. Pat. Nos. 5,336,520;5,505,929; and 5,705,139 and can be practiced as necessary without undueexperimentation.

It has been further discovered according to the present invention thatthe amount of nitrogen in the resulting bulk single crystal can bereduced, not only by using the high purity techniques referred to in theprior art (which are certainly acceptable as part of the inventivetechnique), but also by carrying out the sublimation at relativelyhigher temperatures, while keeping the temperature of the seed crystal,and any bulk growth on the seed crystal at a temperature below thetemperature of the source powder. A preferred technique for sublimationgrowth (other than as modified as described herein) is set forth in U.S.Pat. No. RE 34,861, the contents of which are incorporated entirelyherein by reference (“the '861 patent”).

The sublimation is carried out in an appropriate crucible that, as setforth in the '861 patent, is typically formed of graphite. The crucibleincludes a seed holder, and both are positioned inside of a sublimationfurnace. The SiC source powder is selected and purified as necessary tohave a nitrogen concentration of less than about 1E17 and preferablyless than about 5E16. Furthermore, the source powder has a concentrationof vanadium, or other heavy metals or transition elements, that is belowthe amount that would affect the electrical characteristics of theresulting crystal. Such amounts include those below SIMS-detectablelevels, meaning that using currently available SIMS, they are at leastbelow 1E16 and preferably below 1E14 atoms per cubic centimeter. Thesource powder also preferably meets the other advantageouscharacteristics set forth in the '861 patent.

When a small amount of boron is incorporated as an acceptor, it is bestadded in the form of a source material (powder) that includes thedesired amount.

From a practical standpoint, silicon carbide sublimation can be carriedout with source temperatures ranging from about 2360° C. to about 2500°C. with the temperature of the seed crystal being kept proportionallylower. For the materials described herein, the source was kept atbetween about 2360 and 2380° C. with the seed being 300-350° C. lower.As known to those familiar with such procedures and measurements, theindicated temperatures can depend on how and where the system ismeasured and may differ slightly from system to system.

Because vanadium has been the element of choice for prior attempts toproduce compensated-type semi-insulating silicon carbide, the inventioncan be expressed as the bulk SiC single crystal, and the method ofmaking it, in which vanadium is below the detectable and numericallevels recited above. It will be understood, however, by those familiarwith the growth of silicon carbide and the characteristics of siliconcarbide as used for semiconductor purposes, however, that the inventionlikewise contemplates the absence of any other elements which wouldcause the same functional characteristics (and potential disadvantages)as vanadium.

By avoiding the use of such elements, the invention likewise eliminatesthe need to compensate such elements with other elements andcorrespondingly reduces the complications that such compensationintroduces into the crystal growth processes.

FIGS. 1 through 17 illustrate various measurements carried out on thesemi-insulating substrates according to the present invention, alongwith some comparisons with more conventional compensated anduncompensated silicon carbide material.

FIGS. 1 through 3 represent a corresponding set of measurements taken ona substrate wafer grown at Cree Research Inc., Durham, N.C., inaccordance with the present invention. As set forth in the“Experimental” portion herein, the characteristics of these materialswere tested by the Air Force Research Laboratory in Dayton, Ohio. FIG. 1plots the carrier concentration as against reciprocal temperature (withthe concentration being on a logarithmic scale) for a semi-insulatingsubstrate wafer according to the present invention. The slope of theresulting line gives the activation energy which is approximately 1.1electron volts (eV).

FIG. 2 shows that the resistivity increases as the temperature decreasesin a manner consistent with the other expected properties of thesemi-insulating material according to the present invention.

FIG. 3 represents the mobility plotted against the temperature indegrees Kelvin. FIG. 4 is a plot of the natural logarithm (ln) of thecarrier concentration plotted against reciprocal temperature (degreesKelvin). As known to those familiar with these measurements, the slopeof the natural log of the carrier concentration against reciprocaltemperature gives the activation energy. As indicated by the inset boxin FIG. 4, the activation energy for this sample according to theinvention is on the order of 1.1 eV, i.e., consistent with the resultsin FIG. 1. By comparison, and as likewise known to those familiar withsemi-insulating silicon carbide, the activation energy for thesemi-insulating silicon carbide when vanadium is used as the deep leveltrapping element would be about 1.6 eV under the same circumstances.

The data was measured under a magnetic field of 4 kilogauss on a samplethickness of 0.045 centimeters and over a temperature range from about569 K to about 1,012 K.

FIG. 5 is a plot of the natural log of resistivity as against reciprocaltemperature in degrees Kelvin. This data and this plot can likewise beused to determine the activation energy of the semi-insulating siliconcarbide material. The value of 1.05667 eV determined from this plothelps confirm the 1.1 eV activation energy measured earlier. Stateddifferently, the difference between the activation energies as measuredin FIGS. 4 and 5 is within expected experimental limits, and the dataconfirm each other.

FIGS. 6 through 10 represent the same types of measurements and plots asdo FIGS. 1 through 5, but taken from a different sample; specifically adifferent area of the same wafer as that measured for FIGS. 1 through 5.It will accordingly be seen that FIGS. 6 through 8 are consistent withthe results plotted in FIGS. 1 through 3. More specifically, FIG. 9,which is another plot of the natural log of carrier concentrationagainst reciprocal temperature, shows a calculated activation energy of1.00227 eV. Again, this is within experimental limits of the 1.1 eVmeasured earlier.

In a similar manner, FIG. 10 plots the natural log of resistivityagainst the reciprocal temperature and similarly provides an activationenergy of 1.01159, which likewise is within experimental limits of 1.1eV. FIGS. 11 through 13 show results from yet another portion of thewafer, but which are considered less favorable than the results seen inthe prior measurements. In particular, the plot of FIG. 11 fails to forma straight line in the manner desired, and the data is less favorablethan were the earlier results. Similarly, FIG. 14, which plots thenatural log of resistivity against reciprocal temperature shows acalculated activation energy of only 0.63299, a value well removed from1.1 eV, regardless of the experimental uncertainty.

FIGS. 15, 16, and 17 represent the secondary ion mass spectra (SIMS) ofvarious comparative samples and tend to show the elemental impuritiesand other materials in the semi-insulating silicon carbide substrates.FIG. 15 is the SIMS spectra of the semi-insulating silicon carbidematerial according to the present invention and confirms the absence ofvanadium or any other transition metals in the sample. This confirmsthat the activation energy and mid-gap states present in the inventiondo not result from the presence of vanadium or other transition metals.

FIG. 16 is included for comparison purposes and is the SIMS spectrum ofan N-type wafer of silicon carbide that is neither semi-insulating normade according to the present invention, but instead represents aconductive silicon carbide sample. Because no reason exists to includevanadium for N-type substrates, vanadium is absent from the massspectrum.

FIG. 17 provides a comparison of a prior version of semi-insulatingsilicon carbide which is compensated with vanadium. The vanadium peak isstrongly present at approximately 51 atomic mass units in the spectrum.This vanadium peak is conspicuously absent from both FIGS. 15 and 16.

It will be understood, of course, by those familiar with thesematerials, that although the phrase “below detectable amounts,” is anentirely appropriate description of the invention, these amounts canalso be understood as those that are below the amount that affects theelectronic characteristics, and particularly the resistivity, of thesilicon carbide crystal.

Accordingly, in another aspect, the invention comprises asemi-insulating silicon carbide single crystal with donor dopants,acceptor dopants, and intrinsic point defects. In this aspect of theinvention, the number of donor dopants (N_(d)) in the silicon carbidecrystal is greater than the number of acceptor dopants (N_(a)), and thenumber of intrinsic point defects (N_(dl)) in the silicon carbide thatact as acceptors is greater than the numerical difference between thenumber of these donor and acceptor dopants. Further to this aspect, theconcentration of transition elements and heavy metals is less than theconcentration that would affect the electrical properties of the siliconcarbide single crystal and preferably less than 1E16. The resultingsilicon carbide single crystal has a resistivity of at least 5000 Ω-cmat room temperature, preferably at least 10,000 Ω-cm, and mostpreferably 50,000 Ω-cm.

This aspect of the invention also applies to the complementary situationin which the number of acceptor dopant atoms is greater than the numberof donor dopant atoms. In such a case, the number of intrinsic pointdefects that act as donors is greater than the numerical differencebetween the number of the donor impurities and the acceptor impurities.

Stated differently, the shallow n-type and p-type dopants compensateeach other with one or the other predominating to a certain extent. Thenumber of intrinsic point defects in the crystal that are electricallyactivated is greater than the net amount of n-type or p-type dopantatoms that predominate over the other in the crystal. Stated as aformula,

N_(dl)>(N_(d)−N_(a))

where donors predominate over acceptors, or

N_(dl)>(N_(a)−N_(d))

where acceptors predominate over donors. In the first case, the crystalwould be compensated n-type based on dopant atoms. These net donors arecompensated again, however, by acceptor-type point defects to producethe semi-insulating crystal. In the second case, the point defects actas donor type and compensate the net excess of acceptors in the crystal.

As used herein, the term “dopant” is used in a broad sense; i.e., todescribe an atom other than silicon (Si) or carbon (C) present in thecrystal lattice and providing either an extra electron (a donor) or anextra hole (an acceptor). In the invention, the dopants can be presenteither passively or proactively; i.e., the term “dopant” implies neithera “doping” step nor the absence of one.

In a preferred embodiment, the acceptor is boron. In this embodiment,the boron over-compensates the nitrogen, and the point defects act asdonors to over-compensate the boron to produce the semi-insulatingsilicon carbide crystal. The behavior of boron as an acceptor is incontrast to earlier concepts, in which boron was considered to be a deeptrapping element (e.g., commonly assigned U.S. Pat. No. 5,270,554 atColumn 8 lines 49-51). Indeed, boron can produce a trapping level in SiCat 700 meV, but (to date) not reproducibly so. Accordingly, in thepresent invention, boron has been found to be an appropriate acceptordopant for semi-insulating silicon carbide of the type described herein.

In such a preferred embodiment, the silicon carbide is grown underconditions that reduce the active nitrogen concentration to the point atwhich a relatively small amount of boron, preferably about 1E15 ofboron, will make the crystal p-type. By controlling the growthconditions, the point defect concentrations can be brought to about 5E15thus over-compensating the boron and producing the semi-insulatingcrystal. By reducing the concentration of nitrogen, and thecorresponding compensating amounts of boron, the invention avoids theearlier-mentioned disadvantages of transition-metal domination andheavier degrees of doping and compensation. Because crystal growth ofSiC is a relatively sophisticated process, exact parameters can differdepending on local or individual circumstances such as the particulartemperatures used within the appropriate ranges and the characteristicsof the equipment being used. Nevertheless, based on the disclosuresherein, those of ordinary skill in this art can be expected to practicethe invention successfully, and without undue experimentation.

It is expected that the number of point defects can be controlled tosome extent by irradiating silicon carbide with neutrons, high energyelectrons, or gamma rays to create the desired number of point defectsto achieve the results consistent with the formulas set forth above.

Although an exact number of point defects is difficult to measure,techniques such as electron paramagnetic resonance (EPR), deep leveltransient spectroscopy (DLTS), and position annihilation spectroscopygive the best available indications of the numbers present. As furtherset forth herein, Hall effect measurements also confirm the desiredcharacteristics of the crystal.

In another aspect, the invention can be incorporated into activedevices, particularly active microwave devices, that take advantage ofthe semi-insulating silicon carbide substrate. As noted above and asrecognized by those familiar with active semiconductor microwavedevices, the frequency with which a microwave device can operate can besignificantly hindered by any interaction of carriers with thesubstrate, as opposed to the ideal situation when the carriers arelimited to a particular channel and other functional portions of themicrowave device.

The nature of the silicon carbide semi-insulating material according tothe present invention is such that it has excellent performancecharacteristics in the appropriate devices. These include, but are notlimited to MESFETs, certain MOSFETS, and other devices such as thosedescribed in current U.S. patents and pending applications U.S. Pat.Nos. 5,270,554; 5,686,737; 5,719,409; 5,831,288; Ser. No. 08/891,221,filed Jul. 10, 1997; and Ser. No. 09/082,554, filed May 21, 1998, bothfor “Latch-up Free Power UMOS Bipolar Transistor”; Ser. No. 08/797,536,filed Feb. 7, 1997 for “Structure for Increasing the Maximum Voltage ofSilicon Carbide Power Transistors”; Ser. No. 08/795,135, filed Feb. 7,1997 for “Structure to Reduce the On-resistance of Power Transistors”;and International Application. No. PCT/US98/13003, filed Jun. 23, 1998(designating the United States), for “Power Devices in Wide BandgapSemiconductors”; the contents of all of which are incorporated entirelyherein by reference.

EXPERIMENTAL

Two wafers of semi-insulating SiC were examined at the Air ForceResearch Laboratory at Dayton, Ohio (Wright-Patterson Air Force Base),with high temperature Hall effect and SIMS. No understandable resultswere available from one of the wafers (possibly because ofunsatisfactory ohmic contacts), but two Hall samples from the secondwafer both gave the same results, giving a reasonable confidence levelin those results.

Both wafers were insulating at room temperature. The measurable waferwas thermally activated at elevated temperatures and the carrierconcentration was measurable, which is not always possible insemi-insulating material because of the low mobilities due to the hightemperatures involved. The carrier concentration was around 10¹⁵ cm⁻³ at1000K where the resistivity was about 103 Ω-cm. Such carrierconcentration is about one to two orders of magnitude lower than thatseen in conventional semi-insulating material or vanadium doped materialat the same temperature. No fit of the n vs 1 /T curve could be made,however, so the total concentration for the active layer remainedunavailable. The activation energy was around 1.1 eV.

SIMS was carried out on the sample with a high resolution system.Nothing was seen other than some copper near the detection limit alongwith some hydrogen, which was surmised from the height of the mass 47peak. The mass 47 peak was accordingly attributed to SiOH. The mass scanfor the invention along with the scans for two comparative samples areincluded herewith as FIGS. 18-20, respectively. Titanium (Ti) isapparent at around 1×10¹⁶ cm⁻³ in FIGS. 19 and 20, but not in the sampleof the invention (FIG. 18). Vanadium also appears in the standardsemi-insulating sample (FIG. 20) along with the SiOH line indicatinghydrogen.

From these results, the first wafer was considered to be very highpurity material, and is considered insulating because any residualvanadium impurities along with what other defect makes up the 1.1 eVlevel are present in concentrations larger than the sum of the shallowimpurities and so the 1.1 eV levels compensates the shallow impurities.The Fermi level is pinned at the 1.1 eV level, thus making the materialsemi-insulating. The presence of hydrogen, if any, could mean thathydrogen compensation is taking place, but such would not be expected toselectively compensate or neutralize the shallower impurities and notthe deeper levels.

In the drawings and specification, there have been disclosed typicalembodiments of the invention, and, although specific terms have beenemployed, they have been used in a generic and descriptive sense onlyand not for purposes of limitation, the scope of the invention being setforth in the following claims.

That which is claimed is:
 1. A semi-insulating silicon carbide singlecrystal comprising: donor dopants, acceptor dopants, and intrinsic pointdefects in said silicon carbide single crystal; wherein the number ofdopants of a first conductivity type is greater than the number ofdopants of a second conductivity type; and the number of intrinsic pointdefects in said silicon carbide crystal that act to compensate thepredominating first type dopant is greater than the numerical differenceby which said first type of dopant predominates over said second type ofdopant; and the concentration of transition elements is less than1×10¹⁶; said silicon carbide single crystal having a resistivity of atleast 5000 ohm-cm at room temperature.
 2. A silicon carbide singlecrystal according to claim 1 having a concentration of nitrogen atomsbelow 1×10¹⁷ cm⁻³.
 3. A silicon carbide single crystal according toclaim 1 wherein the concentration of vanadium is less than 1×10¹⁶ cm⁻³.4. A semi-insulating silicon carbide crystal according to claim 1wherein said first type dopants are donors, said second type dopants areacceptors and said intrinsic point defects act as acceptors.
 5. Asemi-insulating silicon carbide crystal according to claim 4 whereinsaid acceptors include boron.
 6. A semi-insulating silicon carbidecrystal according to claim 1 wherein said first type dopants areacceptors, said second type dopants are donors and said intrinsic pointdefects act as donors.
 7. A silicon carbide single crystal according toclaim 1 wherein the polytype of the silicon carbide is selected from thegroup consisting of the 3C, 4H, 6H and 15R polytypes.
 8. A siliconcarbide single crystal according to claim 1 wherein the concentration ofnitrogen is 5×10¹⁶ cm⁻³ or less.
 9. A silicon carbide single crystalaccording to claim 1 wherein the concentration of vanadium is below thelevel that can be detected by secondary ion mass spectroscopy (SIMS).10. A silicon carbide single crystal according to claim 1 wherein theconcentration of vanadium is less than 1×10¹⁴ cm⁻³.
 11. A siliconcarbide single crystal according to claim 1 having a resistivity of atleast 10,000 Ω-cm at room temperature.
 12. A silicon carbide singlecrystal according to claim 1 having a resistivity of at least 50,000Ω-cm at room temperature.
 13. A transistor having a substrate thatcomprises the bulk single crystal according to claim
 1. 14. A transistoraccording to claim 13 selected from the group consisting of:metal-semiconductor field-effect transistors, metal-insulator fieldeffect transistors, and high electron mobility transistors.
 15. Asemi-insulating bulk single crystal of silicon carbide having aresistivity of at least 5000 Ω-cm at room temperature and aconcentration of trapping elements that create states at least 700 meVfrom the valence or conduction band that is below the amount thataffects the electrical characteristics of the crystal.
 16. A siliconcarbide single crystal according to claim 15 having a concentration ofnitrogen atoms below 1×10¹⁷ cm⁻³.
 17. A silicon carbide single crystalaccording to claim 15 wherein the concentration of vanadium is less than1×10¹⁶ cm⁻³.
 18. A silicon carbide single crystal according to claim 15wherein the polytype of the silicon carbide is selected from the groupconsisting of the 3C, 4H, 6H and 15R polytypes.
 19. A silicon carbidesingle crystal according to claim 15 wherein the concentration ofnitrogen is 5×10¹⁶ cm⁻³ or less.
 20. A silicon carbide single crystalaccording to claim 15 wherein the concentration of vanadium is below thelevel that can be detected by secondary ion mass spectroscopy (SIMS).21. A silicon carbide single crystal according to claim 15 wherein theconcentration of vanadium is less than 1×10¹⁴ cm⁻³.
 22. A siliconcarbide single crystal according to claim 15 having a resistivity of atleast 10,000 Ω-cm at room temperature.
 23. A silicon carbide singlecrystal according to claim 15 having a resistivity of at least 50,000Ω-cm at room temperature.
 24. A transistor having a substrate thatcomprises the bulk single crystal according to claim
 15. 25. Atransistor according to claim 24 selected from the group consisting of:metal-semiconductor field-effect transistors, metal-insulator fieldeffect transistors, and high electron mobility transistors.